The invention relates to a method for cooling and calibrating a section of plastic, where the section is introduced, via a matched opening, into a preferably evacuated housing such as a tank or bath, comprising several panels arranged one behind the other, and treated with a cooling medium, for example a liquid, by means of jets arranged peripherally around the section. The invention further relates to a device for cooling and calibrating a section made of plastic, comprising an elongated and preferably evacuatable housing such as a bath or tank, in which several panels are arranged one behind the other each with an opening matching the section or its outer contour and through which the section can be introduced, as well as several jets arranged along the transport path of the section and applying a cooling medium, for example a liquid, to the section.
To cool and calibrate a section of plastic coming from an extruder or extrusion die, the section can first pass along a vacuum drying calibration area and then be cooled using cooling liquid, with calibration taking place at the same time.
For cooling and calibration, full bath and spray bath devices are known. In the case of full bath cooling, the section is passed through a tank filled with water. With spray bath cooling, the section is sprayed on all sides with water, this water being sprayed with an absolute pressure between 2 and 4 bars using jets which can have a diameter of 1 mm or more. The jets themselves are preferably formed by slots provided in tubes extending along the transport path of the section.
Even if spray bath cooling permits an intensive and even cooling of the sections even in areas with undercuts, it entails considerable drawbacks, in particular due to the high water consumption. For that reason, full bath cooling has been employed with increasing frequency recently, as described for example in EP 0 659 536 B1, in DE 295 21 996 U1 or in general terms in the literature DE-Z: Plastverarbeiter, 30. Jahrgang, 1979, No. 6, p. 322-328. However, full bath cooling has the drawback that when the liquid is added surges can arise that can lead to an undesirable deformation of the sections to be cooled and calibrated. In the case of vacuum calibration, fluctuations in the vacuum occur in addition, as a result of which the section to be cooled and calibrated in the full bath might also undergo unwelcome geometry changes.
From DE 32 41 005 A1, a method and a device are known for cooling hollow sections whereby from the start of every extrusion operation water is conveyed into the interior of the hollow section with a pressure between 2.5 and 4 bars, and compressed air with a pressure between 4.5 and 6 bars, hence achieving a micro-fine distribution of the water over the inner surface of the section in order to achieve cooling.
In a device for calibrating extruded sections made of plastic in accordance with DE 34 27 277 A1, hollow sections are passed through a bath having spray jets arranged in the area of its four corners, via which jets the cooling liquid is applied to the section.
Here the bath can be designed as a closed box and evacuated in order to prevent unwelcome deformation of the section.
The problem underlying the present invention is to develop a method and a device of the type mentioned at the outset in such a way that the advantages of both spray bath cooling and full bath cooling are achieved while avoiding all their drawbacks.
In accordance with the invention, the method proposed for a solution to the problem is substantially for the cooling medium to be supplied with a pressure p of pxe2x89xa75 bars to the jet, which in particular has a passage opening for the medium with a diameter d of dxe2x89xa60.7 mm. In particular, the pressure of the cooling medium, for example a liquid, is 10 bars or more compared with the atmospheric pressure and the diameter of the jet is less than 0.5 mm. With these parameters, comprehensive spraying can take place of the section to be cooled and calibrated, without the high water consumption otherwise necessary in spray bath devices. Instead, the water required for facilities of equal dimensions is even less than with the full bath method. For example, comparative measurements have shown that with the same dimensions and throughput in a facility using the full bath method, twice as much water is needed in comparison with cooling in accordance with the invention. In the case of a spray bath cooling system also with the same dimensions and to the prior art, where the liquid is sprayed with a pressure of between 1-3 bars above atmospheric pressure , the water consumption is even 3 to 4 times higher.
In a further embodiment of the invention, it is proposed that the liquid be sprayed by the jets in such a way that an opening spray angle xcex1 transverse to the longitudinal direction of the section of 70xc2x0xe2x89xa6xcex1xe2x89xa690xc2x0 is obtained, in particular 75xc2x0xe2x89xa6xcex1xe2x89xa685xc2x0. This ensures that the section is sprayed peripherally to the required extent and cooled evenly. The latter is ensured in particular by the fact thatxe2x80x94unlike with full bath coolingxe2x80x94an accumulation of air bubbles on the surface of the section is ruled out, as a result of which bubbles the necessary heat dissipation in these areas would not otherwise be assured. In addition, the water used for cooling is always fresh, an advantage which full bath cooling obviously cannot offer. Instead of fresh water, water collected in the bath, which can be passed through a cooling circuit, can also be used.
For intensive and even cooling, it is further provided that at least one jet each is arranged in the area of each comer of a square surrounding the section and is aligned on the section in such a way that normal lines extending from the passage openings of the jets arranged in the comers intersect in an area which is also intersected by a straight line on which lie the respective center points of the panels arranged one behind the other.
The invention further provides that the liquid is sprayed through three jets arranged in straight lines running vertically along opposite sides of the section and/or through two jets arranged in straight lines running horizontally along opposite sides of the section. Naturally the vertical arrangement and the horizontal one can be changed over and/or three jets arranged along each side, for example.
A device of the type mentioned at the outset is distinguished in that the jet has an opening with a maximum diameter d of dxe2x89xa60.7 mm and is connected to a liquid supply in which the liquid is under a pressure p of pxe2x89xa75 bars. In particular, the opening of the jet has a maximum diameter d of dxe2x89xa60.5 mm. The jet can in particular be a hole-type jet, if necessary however also a slot-type jet with a slot running transversely to the longitudinal axis of the section.
The liquid itself is in particular subjected to a pressure p of pxe2x89xa710 bars. It is furthermore provided that the jet is designed such that its maximum spraying angle xcex1 is 70xc2x0xe2x89xa6xcex1xe2x89xa690xc2x0, in particular 75xc2x0xe2x89xa6xcex1xe2x89xa685xc2x0, where the sides of the spray angle or cone are aligned on adjacent surfaces of an imaginary square section.
In particular, it is provided that in each case a jet is arranged in one corner of an imaginary rectangle surrounding the section. In addition, a further jet can be provided in the central areas of vertically running edges of a rectangle.
The panels themselves can rest in a known way on supporting gibs extending from opposite walls of the housing in the transport direction of the section, with jets being integrated into said supporting gibs. Alternatively, it is possible to arrange the panels between in particular cylindrical holding elements such as pin-like journals extending from opposite walls of the housing, with at least some of the holding elements incorporating a jet.